In order to provide an understanding of the aspects of the devices described in the present invention, it is necessary to first discuss prior art in the fields of SAW RFID tags, passive wireless coded SAW sensors and SAW sensor-tag interfaces. Much work in this area has been done since the early 1990s, a summary of which is provided below.
SAW RFID Tags—Prior Art:
SAW devices have been used as radio frequency identification device (RFID) tags since the late 1980's and early 1990's. SAW RFID tags are passive responders, wherein an incident RF signal is captured by an antenna attached to the tag, activates the SAW tag, and is re-transmitted as the reflected (S11) response of the SAW device. Traditional single frequency SAW RFID tags consist of a piezoelectric substrate, generally selected to be temperature stable (hence quartz was a commonly preferred substrate), with a single input/output transducer and a set of reflective taps positioned at various delays on either side of the transducer. The reflective taps are positioned in time “slots” that are separated far enough in time to allow the reflections from any two successive slots to be resolved. The S11 response of a traditional SAW RFID tag consists of a sequence of time domain pulses such that in each successive time slot the existence of a pulse is read as a “1” and the absence of a pulse is read as a “0”. If the code is N time slots long, then the number of unique codes is 2N. The use of two-sided single acoustic track device configurations allowed the designer to incorporate more time slots then would be possible with a single-sided layout. In addition to placing constraints on the number of possible codes for a given device size, the time extent of each time slot (with guardbands on each side to prevent misidentification of taps) also limits the time extent of each reflecting tap. This limits the number of reflecting strips in each tap, which reduces the possible reflection for each tap. Another factor limits the number of possible taps as well. Since the taps are located in the same acoustic track, if taps close to the input/output transducer reflect SAW energy, that removes SAW energy from the wave propagating further out in the track, which means that there is less to be received and reflected from the taps farther out from the transducer. The result is that the signals reflected from sequential taps are of decreasing strength. This effect can be compensated for somewhat by increasing the number of electrodes in the taps that are further from the transducer, but this can introduce significant intertap reflection problems. Intertap reflections occur when the SAW reflected by one tap reflects off of a tap closer to the transducer and propagates once again away from the transducer, only to be reflected from taps further away in the SAW path. In order to avoid intertap reflection problems and allow the SAW energy to propagate and be reflected by multiple taps, it is necessary to keep the total reflectivity of each tap low. Low tap reflectivity, however, results in high reflection loss. The time domain response of typical SAW RFID tags at 2.45 GHz are generally 55 dB or more with approximately 32 taps [1,2] and can exceed 70 dB for devices with many more tap positions. The smaller the number of reflective taps needed to effect the desired number of codes, the lower the possible insertion loss and vice-versa.
In addition to simple on-off coding involving tap positions, a number of other coding techniques have been applied to SAW RFID tags. Phase shift keying has resulted in a higher signal to noise ratio than simple on-off coding, while advanced techniques such as overlapped pulse position modulation combined with phase offsets and multiple pulses per group have been shown to enable larger codesets with adequate signal levels [2,3].
Another method used to avoid intertap reflections and achieve larger codesets is to design devices with multiple parallel acoustic tracks. Input transducers can be connected electrically in series or parallel, with each transducer having a two-sided acoustic track. This reduces the number of reflective taps in each track and allows the designer to use more reflective electrodes in each tap without concern for intertap reflections. However, these techniques also suffer from power division losses in the transducers, and hence cannot substantially reduce insertion loss.
All of the previously discussed SAW RFID tag approaches can be grouped together as reflective delay line techniques. Another type of well known SAW RFID tag uses a resonator approach instead. This type of device consists of multiple SAW resonators that are connected in parallel via matching networks to a common antenna. These very narrowband resonators are designed to be separate in frequency, with guardbands sufficient to ensure that frequencies will not overlap over the operating range of the device. Presence or absence of a response at each frequency constitutes a “1” or a “0” in the corresponding code position. Sensing (for example temperature) can be incorporated in the device by including one or more resonators built on substrates with different characteristics (such as TCD), and monitoring the change in resonance frequency for this resonator measured differentially relative to another reference resonator in the response. [4]
Any of the known SAW RFID tag approaches can be implemented on SAW substrates with properties appropriate for use as sensors. Temperature sensors have been demonstrated [5], and other sensors are possible using similar techniques. Frequency modulated continuous wave (FMCW) interrogation system have been used commercially for temperature sensors on 128 lithium niobate (as in the Baumer-Ident module discussed by Reindl [5]) with total bandwidths of 40 MHz at 2.45 GHz, and pulse position modulation coding enabling 104 codes. In this system, a rough measurement of temperature is obtained by evaluating gross delay, and a finer temperature measurement is obtained by using the gross measurement to eliminate the phase ambiguity of 2π and subsequently utilizing phase to calculate temperature with a resolution of ±2° C. [5]. This approach is extended by Kuypers [6] to achieve accuracies of approximately ±0.1° C.
A wide range of additional work has been done in the area of passive wireless SAW sensor/tag devices, including [7-9]. Reference [10] provides an analysis of the performance achievable using several of the aforementioned algorithms.
Passive Wireless Coded SAW Sensors—Prior Art:
Previously described passive wireless SAW sensors without coding utilize either use the frequency of a resonator or the time delay of a delay line as the parameter for indicating a measured quantity. Multiple sensors could be used in a single wireless system, provided the techniques of frequency diversity or time diversity were used, i.e. multiple resonators at different frequencies or multiple delay lines with different delays or different differential delays were used. In such systems, it was necessary to design these devices with enough frequency separation or enough time separation that the responses of individual sensors would not overlap even when they experience variations in the sensed parameter(s). This necessarily limited the number of potential sensors dramatically, as both time delay and bandwidth are limited.
In the past few years, the inventors have been working with Dr. Don Malocha at the University of Central Florida on development of another type of SAW sensor. Initially envisioned as an Orthogonal Frequency Coded (OFC) SAW sensor, these devices would operate passively, responding to an RF interrogation signal with a reflected signal containing the ID code of the sensor and the measurement of the sensed parameter. Original OFC devices utilized a wideband input/output transducer, and frequency coded reflective arrays in a differential delay line configuration to provide both sensing capability and a unique ID code. The inventors have developed various different embodiments in the past few years, incorporating time and frequency diversity, and modifying the requirements for utilization of strictly orthogonal frequency bands, which are described elsewhere [11]. All of these devices benefit from the use of spread spectrum techniques, whereby a wide frequency band is utilized to interrogate the sensor, and correlation of the sensor response is used to extract both the sensor ID and the measured data with additional processing gain providing higher accuracy of delay measurement than would be achievable through single frequency approaches.
In addition to these approaches, Reindl [12] and Benes [13] both demonstrated that the use of chirp-radar-like techniques in SAW sensors can enhance device sensitivity. Reindl utilized a two-tap delay line with a wideband transducer feeding two chirped reflectors, where the reflectors had opposite chirp sense. Compression of the sensor response in the interrogator and evaluating the difference between the upchirp and the downchirp response provided an increase in sensitivity by a factor of 10 to 100. Benes stated that compression of spread spectrum signals in SAW matched filters (as used in radar systems) can be used to discriminate between different sensors operating in the same frequency band (i.e. to code the sensors), and that using chirp interrogation signals (of traditional SAW RFID devices) with time inverse SAW matched filters in the receiver can increase the effective delay of the SAW sensor and enhance measurement resolution (0.1 mK for temperature is cited for lithium niobate sensors). Ostermayer also discussed CDMA coding techniques for multiple sensor systems using SAW devices [14].
SAW Sensor-Tag Wireless Interfaces:
Another application for SAW devices is to function as a wireless interface to other passive sensors. Brocato demonstrated that a SAW differential delay line could be used, with a sensor that changes impedance with measured quantity attached electrically in parallel with a reflector in one of the paths, to measure variations in the attached sensor [15] Other researchers have also demonstrated similar devices [7, 16, 17]. In one case, operation of a switch was demonstrated. In this example, the devices exhibited a significant amount of self resonance, and what was observed and measured was the amplitude of the reflective response as it rings down. Ringing in the electronic circuit and the direct RF reflection complicates the interpretation of the response signal in this approach. Reading of the reflected signal is not possible until the direct reflections have died out.
Summary of Limitations of Prior Art:
In conclusion, a significant problem with the conventional SAW RFID tag is the high reflection loss. A problem with current passive wireless coded SAW sensors is the limited number of sensor codes that will operate simultaneously. Significant problems with current SAW sensor-tag interfaces include difficulty in interpreting the reflected device response. The present inventions are intended to address each of these issues.